Diffusion in the context of cement and concrete refers to the movement of ions and molecules within the cementitious materials through the pore spaces and microcracks. This process is critical in determining how substances such as water, chlorides, sulfates, and other deleterious agents penetrate and react with the internal components of concrete. The rate of diffusion is influenced by the porosity, permeability, and the microstructural characteristics of the concrete. Proper understanding of diffusion mechanisms helps in predicting the durability and service life of concrete structures, especially in aggressive environments.